[Commonizer] Implement integration test for KT-52050

The test project will be compiled and proves that
'platform.posix.DIR' retains it's 'CPointed' supertype.

The test is expected to fail on platforms that do not support
macos (windows and linux), since the issue seems to be caused
by optimistic, single platform propagation.

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/CommonizerHierarchicalIT.kt

@@ -153,6 +153,16 @@ class CommonizerHierarchicalIT : BaseGradleIT() {
         }
     }
 
+    @Test
+    fun `test KT-52050 - DIR retains CPointed supertype`() {
+        with(Project("commonize-kt-52050-DIR-supertype")) {
+            build(":compileCommonMainKotlinMetadata") {
+                assertNotContains("Unresolved reference")
+                assertSuccessful()
+            }
+        }
+    }
+
     private object Os {
         private val os = OperatingSystem.current()
         val canCompileApple get() = os.isMacOsX

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/commonize-kt-52050-DIR-supertype/build.gradle.kts

@@ -0,0 +1,24 @@
+plugins {
+    kotlin("multiplatform")
+}
+
+repositories {
+    mavenLocal()
+    mavenCentral()
+}
+
+kotlin {
+    macosX64("macos")
+    linuxX64("linux")
+    mingwX64("windows")
+
+    val commonMain by sourceSets.getting
+    val macosMain by sourceSets.getting
+    val linuxMain by sourceSets.getting
+
+    val unixMain by sourceSets.creating
+
+    unixMain.dependsOn(commonMain)
+    linuxMain.dependsOn(unixMain)
+    macosMain.dependsOn(unixMain)
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/commonize-kt-52050-DIR-supertype/src/commonMain/kotlin/CommonMain.kt

@@ -0,0 +1,17 @@
+@file:Suppress("UNUSED_PARAMETER")
+
+import kotlinx.cinterop.CPointed
+import platform.posix.DIR
+
+fun requireCPointed(value: CPointed): Unit = error("Not supported")
+fun DIR(): DIR = error("Not supported")
+
+fun main() {
+    /*
+    Linux and Windows Hosts failed to retain
+    CPointed supertype of `DIR` when commonizing.
+    This happened in the special case of 'single platform propagation', which
+    lost forward declarations from one step to the next.
+     */
+    requireCPointed(DIR())
+}